This invention relates to a differential device for use in a vehicle.
In differential devices disclosed in U.S. Pat. Nos. 3,735,647, 3,884,096, 4,491,035 and 4,750,383 and Japanese Laid-Open (Kokai) Patent Application No. 50625/74, a drive torque applied to those wheels subjected to a larger surface resistance can be made greater than those wheels subjected to a smaller surface resistance. Here, the surface resistance means .mu..multidot.W where .mu. represents a frictional coefficient of the road surface, and W represents a load applied to tires.
Explanation will be made with respect to the case where the above conventional differential device is used as a center differential device for a four-wheel drive vehicle. The conventional differential device comprises a housing to which the torque is transmitted from an engine, and a pair of propeller shafts (front and rear propeller shafts) extending into the housing. The front propeller shaft is operatively connected to front wheels whereas the rear propeller shaft is operatively connected to rear wheels. The pair of propeller shafts extend along a common straight line, and one ends of the two propeller shafts are disposed in spaced, opposed relation to each other within the housing. A pair of side gears (sun gears) are connected respectively to the one ends of the two propeller shafts by splines, and therefore a transmission of rotation between each propeller shaft and its mating side gear is achieved. A screw gear portion is formed on the outer periphery of the side gear. At least one pair of element gears (planetary gears) are rotatably mounted within the housing. The axis of rotation of the element gear is skew relative to the axis of rotation of the propeller shaft and the axis of rotation of the side gear. Each element gear has a screw gear portion and coupling gear portions. The screw gear portion of each element gear meshes with the screw gear portion of the mating or corresponding side gear, and the coupling gear portions of the two element gears mesh with each other. A differential gear is constituted by the two side gears and the two element gears meshing with the side gears and also with each other.
The pair of side gears are of the same configuration, and also the pair of element gears are of the same configuration.
Reference is now made to how the drive torque transmitted to the housing is distributed by the above center differential device to the pair of propeller shafts, that is, to the front and rear wheels.
First, explanation will be made with respect to a condition in which the surface resistance of the front wheels is equal to the surface resistance of the rear wheels or a condition in which the drive torques of the front wheels and the rear wheels are smaller than a critical value causing a wheel slip (This condition will be hereinafter referred to as "basic condition"). In this condition, the drive torque transmitted to the front wheels are equal to the drive torque transmitted to the rear wheels, and therefore this torque distribution ratio is 50:50 if the torque applied to the housing is represented as 100.
Next, explanation will be made with respect to a condition in which the surface resistance of the front wheels is small, so that assuming that a half of the drive torque of the housing is applied to the front wheels, the front wheels is subjected to slip. In this case, the drive torque of the front wheels is reduced to a value smaller than the critical value causing a wheel slip whereas the drive torque of the rear wheels is increased.
In contrast to the above case, when the surface resistance of the rear wheels decreases, the drive torque of the front wheels increases whereas the drive torque of the rear wheels decreases.
The above torque distribution ratio in accordance with the surface resistance varies in the range of between 25:75 and 75:25.
In the above center differential device, theoretically, the torque distribution ratio can be varied over a wider range, for example, of 10:90 and 90:10. However, if the variation range of the distribution ratio is increased, the differential function may fail to be properly performed, and undue forces develop in the device, which shortens the service life of the device. For these reasons, generally, the above variation is in the range of between 25:75 and 75:25 or around such value.
However, in the case where the above conventional center differential device is used in a vehicle such as a truck in which a greater load acts on the rear side of the vehicle when the vehicle is fully loaded with freight, it is often not possible to apply a sufficient propelling force to the vehicle. More specifically, in such a vehicle having a greater load acting on the rear side thereof, under the normal road surface condition, the surface resistance of the rear wheels is large whereas the surface resistance of the front wheels is small. Particularly, at the time of an abrupt start of the vehicle or at the time of going up a slope, a still greater load acts on the rear side, so that the surface resistance of the rear wheels greatly differs from the surface resistance of the front wheels. In this case, the drive torque which can be transmitted to the front wheels without causing slip becomes very small with the result that the drive torque transmitted to the rear wheels is correspondingly decreased. The reason is that the upper limit of the distribution ratio is 25:75, and therefore the drive torque of the rear wheels is only three times larger than the drive torque of the front wheels. As a result, it is not possible to apply a sufficient propelling force to the vehicle.
Under the above circumstances, it has been desired to develop a center differential device for the above vehicle, in which the torque distribution ratio of the rear wheels to the front wheels is increased as much as possible, while securing a smooth differential function and a long service life.
In the above-mentioned differential device, the side gear is of a cylindrical shape, and the screw gear portion is formed on the outer periphery thereof, and a spline portion is formed on the inner periphery. The screw gear portion and the spline portion are disposed at the same position in the axial direction of the side gear, and this arrangement has offered the following problems.
The spline portion is required to have a sufficiently large diameter so that the spline portion itself can have a strength commensurate with the drive torque to be transmitted by the spline portion, and also that the propeller shaft received in the side gear can have a sufficient strength. Also, the peripheral wall on which the spline portion is formed is required to have a sufficient thickness to have a required strength. For these reasons, the screw gear portion disposed radially outwardly of the spline portion is increased in diameter, and therefore the element gear in mesh with the screw gear portion is remoter from the axis of rotation of the side gear. As a result, the housing containing the side gears and the element gears, as well as a differential case containing the housing, is increased in diameter. Because of such increased diameter of the differential device, it has been necessary either to increase the height of the vehicle or to reduce the vertical dimension of the passenger room or cabin of the vehicle.